Various types of brakes, such as electric, air, drum, and disk, have been developed for various types of vehicles and uses. Air brakes, for example, are commonly used on heavy-duty trucks. Drum brakes have been commonly used on other automobiles where such heavy loads are not involved, but federal laws requiring disk-type brakes on new cars have caused the disk-type brake to become more popular. Disk-type brakes are usually less subject to water fading and are typically more resistant to heat fading due to high speed or repeated stops than drum brakes. Also, disk-type brakes are capable of straight-line stops so that hard braking effort does not cause the automobile to pull to the right or to the left.
Although disk brakes have several advantages over drum brakes, problems have also been discovered during some types of uses of disk brakes. For example, when these type of brakes are used with cars enduring high speeds over a long period of time, such as with race cars, the brake pedal often develops a spongy feeling or brake fade which reduces the brakes' effectiveness and produces less than optimum braking action. The spongy feeling in the brake pedal is typically caused by boiling hydraulic fluid or water in or around the brake caliper. The boiling hydraulic fluid or water, in turn, is caused by the heat generated from the application of the brakes at high speeds.
Some attempts to solve this problem have included forcing air into the caliper area or circulating water through the caliper area to keep the fluid from boiling. These attempts, however, have failed because of the lack of understanding of the braking system operation.
Other attempts recognized that some of the brake fade is caused by the brake fluid remaining in the caliper. Therefore, attempts to solve the problem based on this recognition have arranged check valves either within or closely adjacent the disk brakes to allow fluid to circulate through the brake caliper. An example of such a disk brake, according to U.S. Pat. No. 4,799,575 by Kroniger entitled "Hydraulically Controlled Disk Brake With Serial Fluid Flow" is shown in FIG. 1.
The braking system 10 of FIG. 1 has a master cylinder 11 which includes a fluid reservoir 12 and a brake pedal 15 connected to the master cylinder 11 for actuation of the braking system 10. A pair of hydraulically controlled disk brakes 20 are connected to the master cylinder 11 by brake lines 30. In operation, the fluid flows along brake line region 31 and into brake line region 32. The fluid flows through a first check valve 25 and into brake line region 33. Fluid is prevented from flowing into brake line region 34 by a second check valve 26. The fluid flows from brake line 33, down into the caliper 24, and to a plurality of pistons 22 and cylinders 23 guiding the pistons 22. The fluid then flows through brake line region 36, back to the pistons 22 and cylinders 23, and into brake line region 34. The fluid further flows through the second check valve 26 through brake line region 35 and back to the master cylinder 11. This attempt, although allowing some fluid to circulate through the brake caliper 24, failed to completely solve the brake fade because air or gas continues to accumulate within the brake lines 30 of the braking system 10.
Thus, there is a need for a braking system that provides circulation of fluid through the brake caliper and purges the brake lines of air or gas therein.